The present invention relates to a dual pump traverse and feed system for controlling the operation of a reciprocatory hydraulic motor in the form of a single rod piston operatively mounted in a three chamber cylinder.
A common requirement in many hydraulic systems is that the piston of a reciprocatory fluid motor be driven at relatively high speed up to a certain point in its forward stroke, and then be driven through the remainder of its forward stroke at a relatively low speed under a relatively high applied pressure. A standard system for accomplishing this result is a so called "high low" system which employs two pumps, one of which can displace a relatively high volume of fluid at relatively low pressure and the other of which can displace a relatively low volume of fluid under high pressure. Because of its limited displacement, the low volume pump is unsuitable for rapidly driving the piston, even when there is no substantial resistance to movement of the piston, while the high volume pump has sufficient capacity to drive the piston rapidly, but cannot apply any substantial amount of pressure to the piston unless it is driven by a relatively high horse power motor. The high low system minimizes the power requirements by connecting the output of both pumps to drive the piston in rapid traverse and then disconnecting and dumping the output of the high volume pump and driving the piston through the final portion of its stroke by the low volume, high pressure pump. Such systems are typically operated in an open loop circuit.
The single rod three chamber motor referred to above includes two chambers within the piston to which equal area piston surfaces are exposed at the rod end side and the head end side of the piston. A third chamber hydraulically isolated from the first two chambers is exposed to a third area on the piston which faces the head end side of the piston. This particular type of motor is well adapted to a rapid traverse, low speed feed application in that rapid traverse in either direction is possible by utilizing the first two chambers, and the pressure applied during the feed portion of the forward stroke may be augmented by supplying fluid under pressure to act against the third area.
While the three chamber cylinder described above is readily adapted to closed loop operation during rapid traverse due to the equal areas on the opposed sides of the piston, a closed loop operation during the feed stroke presents a problem in that more fluid is going into the cylinder--to act on both the third area and the rod side equal area--than is coming out from the chamber at the rod end equal area side of the piston.
While rapid traverse and low speed feed can be achieved simply by connecting a variable displacement pump to opposite sides of a simple cylinder-piston motor, the displacement range of the pump may be such that when operated at a minimum displacement, the volume of fluid supplied to the piston frequently may be too high to reduce the velocity of the piston to the desired velocity of the feed.
The present invention is directed to a dual pump system operable to drive a single rod piston of a three chamber cylinder in rapid traverse in a closed loop system and to drive the piston in a low speed forward feed stroke established by the displacement of one of the dual pumps, and to do this also in a closed loop circuit.